So, you know what Games Workshop's Warhammer 40.000 (40K) is, you like it and you're sure you want to start with a not-easy-to-play army such as the Tau Empire? Good! Welcome to the Greater Good. Now let's make sure you don't waste your money on "superfluous" units.

Steps

1

Keep in mind the current Tau strategies are developed according to the Games Workshop's Tau Empire 4th edition Codex and the Games Workshop's 6th edition Rulebook - which will influence your purchases.

2

Look around for places to buy them. There are many good places to buy your items from: Games Workshop's store, independent retailers, eBay, even Craigslist! Sometimes people get rid of their armies too (which is a great way to start playing if you do not want to spend time and money painting.

3

Consider buying a Battleforce. A very good way to start shopping and playing is to buy a Battleforce - you'll get one Battle suit, one Kroot Carnivore unit, one Fire Warrior unit, one Stealth Suit unit, and a precious Devilfish.

4

Get Battle suits. They'll fulfill the roles of HQs (and their Bodyguards), Elite teams and Broadside Heavy Support units. The Battlesuit is the most versatile unit of the Tau Codex. Stealth Suits and Ethereals do not compare. And you need Sky Ray boxes (you want these for the extra sprues and the money value: you'll be actually assembling Hammerheads/Devilfishes; see below).

5

According to the Force Organisation Chart (FOC), you may have up to two HQs, three Elites, three Fast Attack, three Heavy Support and six Troop choices (but a minimum of one HQ and two Troop choices).

6

Start playing. Starting to play Tau with an HQ and two Troops (from the Battleforces) would be a good idea if the bulk of their gameplay was created to use Troops - it was not. The bread-and-butter of the Tau Empire are the Battlesuits. So you'll be looking at about two HQs (both Battlesuits), XV8 Battlesuits as Elites choices (individually or in teams, you'll find your sweet spot) - and troops that suit your strategy and point availability. Later you'll be definitely using TEAMS of Elite Battlesuits, and Bodyguards for the HQs. By then you'll have bought 15 Battlesuit boxes - yeah, start saving for them now.

7

See each of the other Tau units in detail:

The Ethereal (HQ) - they're good in combat and have some interesting abilities but learn on thing: Tau are not melee warriors. So buy them only if you have spare change - to use them as decoration or objective markers, at best.

A quick note on the Battlesuit Commander (HQ): their boxes are more expensive than the average Battlesuit, but you only get two new heads and the Airbursting Fragmentation Projector and the Cyclic Ion Blaster (and no Gun Drone!) - but you can convert them, can't you? Yeah, 'thought so, so avoid theses purchases unless you've got the money to spare.

The Stealth Suits (Elite) - their sprues are awesome for conversions, with some great designs, so don't bother assembling them: they cannot compete with the Battlesuits - who needs more Strength 5 shots/Armor Penetration 5 shots? Still, they are interesting in Kill Team games.

The Fire Warriors - a weak Troop choice, they might be good for securing objectives, or being properly equipped for tank busting, but they're better inside their Devilfishes, held in Reserve for increased protection. Only buy the Shas'ui model if you have the money to spare.

The Kroot Carnivores (Troop) - the best the Tau can recruit for close combat, but definitely not the best there is in the 40K universe. They can secure objectives, infiltrate (and thus also outflank) and shoot. And if you add Hounds, the unit becomes even more powerful - but Hounds are ridiculously expensive; take a look at the Chaos Warhounds and how they look outwardly ferocious... and a bargain! And the Krootox, though powerful, is expensive and limits the Carnivores a little. Again, the Kroot Shaper can be easily modeled.

Soon you'll see how limited the Tau Troop choices are. There's a Chapter Approved article from the White Dwarf magazine describing the Tau Human Auxiliaries, the Tau Gue'Vesa (great for Tank Busting). These are interesting options, and could be modeled from Imperial Guard units, like the Cadians or Catachans, which could be cheap if bought as push-fit models.

Gun Drone Squadron (Fast Attack) - An interesting choice, the whole unit can be assembled from individual Gun Drones that come with each Battlesuit you buy. Still, the Gun Drone can be turned into a Shield Drone by removing the lower Pulse Carbine section or through the GW Shield Drone kit (expensive...) or through a creative conversion. If you buy the Sky Ray vehicle (see below), you can even create your own Markerlight drones.

The Pathfinder Team (Fast Attack) - a very important unit for higher point games, they could be modeled from Pulse Carbine Fire Warrior models. The Rail Gun variant is a nice model though, specially the ones lying down.

The Piranha (Fast Attack) - an important vehicle for screening, these have no alternatives whatsoever (except for Papercraft). Buy one model when you feel the need, probably after you've reached the 1000 point range. To reach their full potential, use full units of three models.

Stingwings (Fast Attack) - a broken, expensive unit (point and money wise), their use is tricky, even in the 6th edition. Besides, they'd be taking one Fast Attack slot from the Pathfinders or the Piranhas.

The Broadside (Heavy Support) - an important Battlesuit, especially now in the 6th edition when the Hammerhead is more vulnerable, and for carrying high-accuracy Railguns. The box comes with the same XV8 Battlesuit plastic sprues, plus feet, additional torso parts, modified arms and the Railguns, all in metal. If only you could make copies of these parts, you could get free Broadsides for the same price as XV8 Battlesuits...

Sniper Drone Team (Heavy Support) - small, powerful and camouflaged, these units are a powerful addition to any army. If you have spare gun drones and rail rifle Pathfinder models, you could convert them into Sniper Drones (a Fire Warrior and the Stealth Suit sprues could be converted into the Spotter).

The Hammerhead (Heavy Support) - after the Devilfish, it's the most important vehicle for the Tau: its Railgun is the most powerful weapon in our codex. Now, they cost the same as the Sky Ray box, and here's the trick: the Sky Ray box comes with ALL the sprues to create a Hammerhead - so why not buy the Sky Ray and have the option to use the tank as a Sky Ray in the future?

The Sky Ray (Heavy Support) - it's a good vehicle, but hardly employed in armies under 2000 point games, especially when you only have three Heavy Support slots and Broadsides, Hammerheads and Sniper Drone teams to fill them...

Special Characters: available only in 1500 point games and up, these are interesting additions to an army - except for Aun'Va: although a cool model (decorative item, perhaps?), you don't want to spend all those points (and HQ slot) with a melee, non-scoring unit, right? Commander Farsight makes for an even more Battlesuit crowded army (yay!) and Commander Shadowsun could not only efficiently Tank Hunt but also boost the morale saves of your entire army!